1. Field of the Invention
The invention relates to air-operated diaphragm brake actuators for vehicles and particularly to spring brake actuator assemblies and the springs used therein.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly that is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Typically, the spring brake actuator is disposed in tandem with the service brake actuator. When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting the operation of the brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
In a typical prior art air brake system, the spring brake actuator and the service brake actuator are disposed in a single housing comprising a spring brake portion and a service brake portion. The service brake portion includes an air chamber partially defined by a flexible service diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake portion includes a spring chamber and an air chamber, both partially defined by a spring brake diaphragm acting against a spring pressure plate to compress the compression spring in the spring chamber when air pressure is applied to the spring brake diaphragm in the air chamber. An actuating rod extends through the spring brake diaphragm to integrally connect with the pressure plate. In operation, it is pushed outwardly from the air chamber through a housing opening and bearing provided with a pneumatic seal to engage the service diaphragm and push rod of the service brake, thereby causing the brake to be applied. The spring brake diaphragm is provided with a centrally disposed aperture having an annular edge and the actuator rod extends through the opening and engages the annular edge to form an airtight seal. The actuator rod is hollow with a central bore and a brake releasing caging bolt extends into the central bore. An end plate on the bolt engages the spring brake pressure plate to draw the spring to a compressed state when the spring is caged by rotation of the bolt.
The spring used in the spring chamber is typically a large force compression spring. The size of the spring depends upon the size of the actuator, but each has a certain represented force curve over the length of stroke. An example of a typical force curve is shown in FIG. 3.
The large force compression spring in an air-operated diaphragm spring brake actuator is under severe compression at all times. For example, in a 30-30 brake actuator, the spring (having an unreleased length of about 8-8xc2xd inches) is held under compression to a length of about 1xc2xd inches when the spring brake actuator is retracted. Normal stroke length of the brake actuator is about 1-1xc2xd inches, so even when the spring is extended, it remains compressed at a maximum length of about 3xc2xd inches. Because of these high internal stresses in the spring, the large force compression springs are typically cold formed at ambient temperature, heat treated, and then subsequently set after cooling (cold set springs). One problem that is commonly associated with cold set springs is that load loss in the spring over time increases, typically to a range of between 10% and 18%.
Heat setting, also known as hot pressing, is a known process for setting a spring at elevated temperature in order to minimize loss of load at operating temperature. One trade-off, however, is that a hot set spring will maintain higher internal stresses for a longer period. Consequently, hot set springs typically lack the durability of cold set springs. It is also known to use hot set springs in motor vehicle suspensions. But suspension springs are not normally under load sufficient to aggravate the normal stresses of a hot set spring, and durability is thus not normally an issue.
There is a desire to make spring brake actuators last longer, but the spring relaxation rates of cold set springs are becoming a significant issue. There is a need to reduce load loss over time in the large force compression spring of a spring brake actuator.
A spring brake actuator according to the present invention addresses the problem of load loss by incorporating a hot set large force compression spring into the actuator while at the same time protecting the spring.
The invention finds itself in an improvement in a brake actuator for a vehicle of the type comprising an enclosed housing with a diaphragm disposed in the housing and dividing the interior into a first chamber and a second chamber, the diaphragm is reciprocally movable in response to the delivery and exhaust of pressurized fluid to and from the first chamber. An actuator rod in the first chamber is movable with the diaphragm to operate a brake. A spring is disposed in the second chamber to urge the diaphragm to a first position where the first chamber is collapsed with the exhaust of pressurized fluid.
In one aspect of the invention, the improvement is having the second chamber sealed and the spring hot set. Preferably, the second chamber has filtered vent openings, although it is within the invention to have the second chamber completely sealed. In another aspect of the invention, the spring is hot set, and the axial motion of the spring is guided to minimize coil clash. In this aspect, the spring is fixed to the diaphragm and the motion of the actuator guides the spring.